In Thunderbird PDF attachments have always opened in Adobe Acrobat or Adobe Reader. With the latest upgrade PDFs now open in Thunderbird itself, and if I want to see them in Acrobat or Reader, I have to first save them to a folder outside Thunderbird and then open them from there, which is very inconvenient, but apparently the only option if I need to compare 2 documents side by side. Yes, Thunderbird shows a tab for each document, but that is not a side-by-side view, which is needed to compare lists of items on vendor invoices. This is not an improvement but instead reduces the functionality of Thunderbird, and is not necessary. Please put it back the way it was. There's nothing worse than technology that has been overthought without the needs of the end user in mind. If it ain't broke, don't fix it.